The present invention relates to a laser device and a method of producing a semiconductor device and, more particularly, to a laser device that allows adjustment of characteristic values of a semiconductor laser element after producing the laser, and a method of producing the laser device.
As the society becomes increasingly dependent on the information technologies, various communications apparatuses are being imposed with requirements to have multimedia capabilities. Thus processing speeds and data handling capacities of these apparatuses have been increased, and attempts have been made for the application of wavelength division multiplexing transmission technology to optical signal transmission over trunk lines such as submarine cables.
In the wavelength division multiplexing transmission, distributed feedback semiconductor laser devices (hereafter referred to as DFB-LD) is used as the light source with 20 to 100 DFB-LDs of different oscillation wavelengths being arranged in an array, each oscillating to emit light at a predetermined wavelength that is transmitted by an optical cable and combined with other light in a coupler. It is therefore important in achieving wavelength division multiplexing transmission to stabilize the oscillation wavelength of each DFB-LD and accurately determine the intervals between the oscillation wavelengths.
A band of wavelengths used in the wavelength division multiplexing transmission is defined according to a recommendation by ITU-T (International Telecommunications Union, Telecommunication Standardization Sector), with the interval between adjacent wavelengths being regulated to be 0.8 nm. Accordingly, it is recognized that the DFB-LD used as the light source preferably oscillates at a wavelength within xc2x10.1 nm of the defined wavelength.
However, the DFB-LD of the prior art has a problem that, since the accuracy of oscillation wavelength has been limited to about 1 nm because the processing margin is insufficient when producing an element while setting a particular wavelength in the processes of crystal growth and wafer processing, use of the DFB-LD as the light source for wavelength division multiplexing transmission has been impractical for the reason of the accuracy of oscillation wavelength.
Sudoh et al. (ELECTRONICS LETTERS; Jan. 30, 1997 Vol.33, No.3, p.216-p.217) have recently reported a DFB-LD that enabled it to tune the oscillation wavelength to a desired value by a method as follows: A part of an optical waveguide of a DFB-LD is provided with a film made of a material that changes refractive index depending on the heat generated by laser irradiation, and the refractive index of the film is changed by irradiating it with a laser beam while measuring the oscillation wavelength of the DFB-LD after the laser element has been made, thereby changing the effective refractive index of the waveguide and achieving the desired oscillation wavelength.
Although demands for laser elements having more accurate oscillation wavelengths are increasing, the demands cannot be satisfied in the production process only. Thus such techniques have been developed that achieve laser elements having the desired oscillation wavelengths by adjusting the oscillation wavelength after producing the laser element.
Adjustment of the characteristic values to be done after producing the laser element includes, in addition to the oscillation wavelength of the DFB-LD, adjustment of other characteristic values such as the photon density in an active layer of a ridge type semiconductor laser.
FIG. 10 is a sectional view showing the structure of a DFB-LD of the prior art.
In FIG. 10, reference numeral 1 denotes an n-InP substrate, 2 denotes an n-InP buffer layer, 3 denotes an n-InGaAsP light confinement layer, 4 denotes an MQW active layer, 5 denotes a p-InGaAsP light confinement layer, 6 denotes a diffraction grating layer, 7 denotes a p-InP first cladding layer, 8 denotes an Fe-doped InP embedding layer, 9 denotes an n-InP embedding layer, 10 denotes a p-InP second cladding layer, 11 denotes a p-InGaAs contact layer, 12 denotes an SiO2 insulating film,.13 denotes a Cr/Au vapor deposited film and 14 denotes an anode of Au-plating layer. 15 denotes a metallic vapor deposited film and 16 denotes a cathode of Au-plating layer provided on the surface of the metallic vapor deposited film.
Oscillation wavelength xcex of the LD having the structure described above is given as follows assuming the effective refractive index neff of the optical waveguide and the interval xcex9 of the diffraction grating.
xcex=2xc2x7neffxc2x7xcex9
When xcex9 is 240 nm and neff is 3.23, for example, then xcex is 1550.4 nm.
Factors that determine the value of neff include the distribution of refractive index of the material in a region from which light leaks out that is a circular area about 2 xcexcm in diameter, and particularly important are composition and film thickness of the n-InGaAsP light confinement layer 3, the MQW active layer 4 and the p-InGaAs light confinement layer 5 that constitute the optical waveguide and the width of the optical waveguide.
However, the DFB-LD shown in FIG. 10 is difficult to produce due to variations in the process, while maintaining uniform conditions for the composition and film thickness of the n-InGaAsP light confinement layer 3, the MQW active layer 4 and the p-InGaAs light confinement layer 5 that constitute the optical waveguide and the width of the optical waveguide that are the factors which determine the value of neff, and therefore it has been difficult to produce the DFB-LD having oscillation wavelength of accuracy within xc2x10.1 nm due to the variation in neff.
One of the solutions for this problem is the wavelength tuning DFB-LD structure proposed by Sudoh et al. described previously.
FIG. 11 is a sectional view showing the structure of the wavelength tuning DFB-LD of the prior art.
In FIG. 11, reference numeral 21 denotes an n+-InP substrate, 22 denotes an n-InP layer, 23 denotes an active layer, 24 denotes a diffraction grating layer, 25 denotes a p-InP layer, 26 denotes a p+-InGaAs layer, 27 denotes Ti/Au electrode, 28 denotes a wavelength control film made of As4Se5Ge1 and 29 denotes an Al2O3 film.
When the wavelength control film 28 was irradiated with light emitted by a He-Ne laser of wavelength 632.8 nm with power density of 1.3 W/cm2, a wavelength shift of 0.14 nm was observed.
FIG. 12 shows an embedding type DFB-LD of wavelength tuning type of the prior art which is a modification of the wavelength tuning DFB-LD structure proposed by Sudoh et al. turned into an embedding type DFB-LD shown in FIG. 10.
In FIG. 12, reference numerals identical with those used in FIG. 10 and FIG. 11 denote the same or corresponding components.
Changes in the refractive index of As4Se5Ge1 that constitutes the wavelength control film 28 due to the wavelength of Ar laser light are reported in the ELECTRONICS LETTERS mentioned previously by Sudoh et al, indicating that maximum change in the refractive index at wavelength 1.55 xcexcm was 0.027.
Thus assuming that width w of the optical waveguide of FIG. 11 is 1.3 xcexcm, total width W of the optical waveguide including the embedding layers is 1.7 xcexcm and thickness of the wavelength control film 28 is 0.5 xcexcm, then the range of neff values of the adjustable effective refractive index determined upon computation of the light propagation mode is from 3.18716 to 3.18728. Consequently, when xcex9 is 240 nm, adjustable range of the oscillation wavelengths is from 1529.84 nm to 1529.89 nm, giving a tunable band of 0.05 nm.
However, when the wavelength control film made of As4Se5Ge1 is used as in the method described above, since the change in the refractive index of As4Se5Ge1 caused by laser irradiation is an irreversible change, refractive index of the wavelength control film made of AS4Se5Ge1 which has once decreased cannot be increased. Therefore, a failure in tuning the wavelength results in a rejected product out of the wavelength standard which cannot be tuned again. Thus there has been such a problem that the tuning operation must be done very carefully, leading to increased time taken which results in an increased production cost of the semiconductor laser element.
The present invention has been made to solve the problems described above.
A first object of the present invention is to provide a laser device that allows it to easily adjust the characteristic values of a laser element, thereby to produce the laser devices having uniform characteristics easily at a low cost.
A second object of the present invention is to provide a distributed feedback semiconductor laser device that allows it to easily adjust the oscillation wavelength and has a predetermined oscillation wavelength of high accuracy.
A third object of the present invention is to provide a ridge type semiconductor laser that allows it to adjust the photon density in an active layer and has a low threshold of oscillation at a low cost.
A fourth object of the present invention is to provide a method of producing a laser device that allows it to easily adjust characteristic values of a laser element, thereby to obtain the laser devices having good characteristics at a low cost.
Prior art for producing laser devices include, besides that shown in FIG. 10 through FIG. 12, for example, a semiconductor laser device disclosed in Japanese Patent Kokai Publication No. 8-116138. It is a semiconductor laser device having a semiconductor laser element sealed in a case that has a laser light emission window, while the element is cooled by circulating a liquid which is transparent to the light of the oscillation wavelength of the laser. The application mentioned above has no description with regards to forming at least a part of layers of the waveguide of the semiconductor laser element to such a predetermined width as light leaks from a side wall of the waveguide, and to adjustment of the oscillation wavelength by means of the fluid that makes contact with this part.
The laser device according to the present invention comprises a semiconductor substrate, a semiconductor laser element made in such a configuration as at least a part of layers of a waveguide that includes a first cladding layer of a first conductivity type, an active layer and a second cladding layer of a second conductivity type disposed successively on the substrate is formed to a predetermined width that allows light to leak from a side wall thereof along the direction of light propagation, a case that houses the semiconductor laser element disposed therein, the case surrounding the side wall of the waveguide and having an aperture capable of letting the fluid flow in and then sealing the case, and a fluid that is sealed in the case to make contact with the side wall and has a predetermined refractive index, wherein intensity of light leaking from the side wall can be regulated by changing the refractive index of the fluid contacting the side wall of the waveguide, thus making it possible to adjust the characteristics values of the laser element.
According to the present invention, the waveguide is further provided with a diffraction grating layer, so that the effective refractive index of the optical waveguide can be controlled and the oscillation wavelength of the DFB-LD can be adjusted by changing the refractive index of the fluid that makes contact with the side wall of the waveguide.
Also a part of thickness of the second cladding layer of the waveguide is formed into a ridge that has a predetermined width which allows light to leak, so that the photon density in the active layer of the ridge type laser can be adjusted by regulating the intensity of light leaking from the second cladding layer.
Also the laser element is disposed in the case that surrounds the side wall and has a first part comprising the aperture capable of letting the fluid flow in and then sealing the case and a second part having an emission window provided therein to oppose an light emitting end face of the laser element, the first part and the second part being disposed to interpose a partition wall that seals off the fluid, so that there will occur no decrease in the emission efficiency and in the single-mode oscillation performance since the laser light emission end face does not make contact with the fluid that has the predetermined refractive index and the reflectivity of the emission end face does not change.
Moreover, the laser element is disposed in the case and walls that constitute a same chamber of the case have the emission window disposed to oppose the light emitting end face of the laser element and the aperture capable of letting the fluid flow in and then sealing the case, thereby making the constitution simple.
Furthermore, silicon oil is used as the fluid that has high heat conductivity, and therefore thermal stability of the laser device is improved.
The method of producing the semiconductor device according to the present invention includes a process of operating the semiconductor laser element, letting fluids of different refractive indices successively through the aperture into the case, and adjusting the characteristic values while measuring the characteristic values of the laser element, and therefore the characteristic values of the laser element can be adjusted in a reversible manner, thus resulting in improved yield of production.
Also because an inlet aperture and an outlet aperture are provided so that fluids of different refractive indices can be caused to continuously flow in and out through the inlet aperture and the outlet aperture while adjusting the characteristic values, thus time taken in the adjustment can be reduced.